Drop on demand inkjet technology for producing printed media has been employed in commercial products such as printers, plotters, and facsimile machines. Generally, an ink image is formed by selectively ejecting ink drops from a plurality of inkjets, which are arranged in a printhead or a printhead assembly, onto an image receiving surface. For example, the printhead assembly and the image receiving surface are moved relative to one other and the inkjets are operated to eject ink drops onto the image receiving surface at appropriate times. The timing of the inkjet activation is performed by a printhead controller, which generates firing signals that activate the inkjets to eject ink. The image receiving surface may be an intermediate image member, such as a print drum or belt, from which the ink image is later transferred to a print medium, such as paper. The image receiving surface may also be a moving web of print medium or a series of print medium sheets onto which the ink drops are directly ejected. The ink ejected from the inkjets may be liquid ink, such as aqueous, solvent, oil based, UV curable ink or the like, which is stored in containers installed in or near the printer. Alternatively, the ink may be loaded in a solid form that is delivered to a melting device, which heats the solid ink to its melting temperature to generate liquid ink that is supplied to a print head.
During the operational life of these imaging devices, inkjets in one or more printheads may become unable to eject ink in response to a firing signal. These inoperative inkjets are also called malfunctioning inkjets or ejectors. Because printing systems typically have on the order of fifty to one hundred thousand inkjets, some malfunctioning inkjets are almost always present in the system. The defective condition of the inkjet may be temporary and the inkjet may return to operational status after one or more image printing cycles. In other cases, the inkjet may not be able to eject ink until a purge cycle is performed. A purge cycle may successfully unclog inkjets so they are able to eject ink once again. Execution of a purge cycle, however, requires the imaging device to be taken out of its image generating mode. Thus, purge cycles affect the throughput rate of an imaging device and are preferably performed during periods in which the imaging device is not generating images. The printing device must be able to function routinely with some number of malfunctioning inkjets.
Methods have been developed that enable an imaging device to generate images even though one or more inkjets in the imaging device are unable to eject ink. These methods cooperate with image rendering methods to control the generation of firing signals for inkjets in a printhead. Rendering refers to the processes that receive input image data values in one form convenient to the user or upstream portion of the system and then process these received data accurately into output image data values that express the image in another form convenient for the downstream system, typically an electromechanical marking engine. The output image data values are used to generate firing signals for printheads to cause the inkjets to eject ink onto the recording media. Once the output image data values are generated, a method may use information regarding defective inkjets detected in a printhead to identify the output image data values that correspond to a defective inkjet in a printhead. The method then searches to find a neighboring or nearby output image data value that can be replaced to compensate for the defective inkjet. Another method is able to compensate for the defective inkjet because a normalization process may be used to establish a maximum output image data value for inkjets that is less than the output image data value that causes an inkjet to eject the maximum amount of ink that can be ejected by an inkjet. Thus, an output image data value can be increased beyond the normalized maximum output image data value to enable an inkjet to eject an amount of ink corresponding to the maximum output image data value plus some incremental amount. By firing several nearby inkjets in this manner, the ejected ink density can approximate the ink mass that would have been ejected had the defective inkjet been able to eject the ink for an output image data value that corresponds to the defective inkjet. Another method may rely on the configuration of printheads in the printer that enables inkjets that eject ink drops of different colors at a same position on the substrate that receives the ink drops. When one of these inkjets that eject drops at the same position malfunctions, some of the ink that would have been ejected by the malfunctioning inkjet is provided by ejecting drops from one of the other functioning inkjets that eject ink onto the same location that the malfunctioning inkjet would eject ink.
The methods that compensate for malfunctioning inkjets by adding all or a portion of an output image data value associated with the malfunctioning inkjet to output image data values associated with nearby operating inkjets in the same printhead are effective. The ink drops ejected by the nearby inkjets are reasonably well aligned with the other ink drops in the same area since they are ejected by the same printhead. Additionally, the ink drops are the same color as those ink drops that would be ejected by the malfunctioning inkjet. This method requires, however, that approximately 20% of the inkjets nearby the malfunctioning inkjet have zero or nearly zero output image data values in order for a sufficient number of locations be available for accepting all or a portion of the output image data value for the malfunctioning inkjet to mask the absence of the ink to be ejected by the malfunctioning inkjet. Consequently, this method is not capable of compensating for malfunctioning inkjets that would eject ink drops into high density regions of an image. The method that ejects ink drops from another printhead that is different than the printhead containing the malfunctioning inkjet can eject ink drops in high density areas at the position where the malfunctioning inkjet would eject ink drops; however, the effectiveness of this approach is degraded because the ink drops from the inkjets in the alternative printhead are imperfectly aligned with the malfunctioning inkjet. Thus, a thin line of high optical density contrast tends to occur, which produces a visible streak in the final image. Additionally, the compensating ink is not the same color as the ink that would be ejected by the malfunctioning inkjet. Because the human visual system is less sensitive to high frequency variations in hue and saturation than it is to variations in intensity, this color imperfection is preferred to marking nothing at all in the position of the malfunctioning jet. Nevertheless, these compensating ink drops are a different color than the ink drops that would be ejected by the malfunctioning inkjet and may affect the hue and color saturation of the image in a humanly perceptible manner. Therefore, developing a compensation scheme for malfunctioning inkjets that enables compensation in high density areas without producing humanly perceptible or objectionable image quality issues would be useful.